The present invention relates to apparatus for drilling an undersea borehole and, more particularly, to guide base apparatus forming a foundation on the ocean floor for drilling activity. Still more particularly, the present invention relates to a method and apparatus for recovering all such guide base apparatus in a single trip from the ocean floor to an offshore drilling platform.
The development of oil producing capability in offshore oil fields often includes, at preliminary stages, the drilling of multiple boreholes which are not intended to support producing wells. Such boreholes might be drilled, for example, for exploration of geological formations thought to contain hydrocarbon fluids or for appraisal of known hydrocarbon-containing formations to determine the optimum positions for producing wells.
Drilling an exploratory or appraisal well begins with lowering a temporary guide base from the offshore drilling platform floating at the ocean surface down to the ocean floor. The temporary guide base is a foundation structure which includes a central opening and four guidelines which extend from the offshore platform to positions equally spaced about the guide base. A utility guide frame is attached to the guidelines at the platform and used to lower drilling tools down to the temporary guide base and through the central opening therein to the ocean floor. Using the drilling tools and seawater, a pilot hole is drilled through the temporary guide base to a depth of 100 to 600 feet. Return fluid with drilled cuttings is spilled onto the temporary guide base and surrounding ocean floor.
After the pilot hole has been drilled, a string of thirty inch casing, sometimes called a foundation pile, is lowered through the temporary guide base into the pilot hole and is cemented into place. The upper end of the foundation pile includes a conductor housing assembly which supports the foundation pile from the temporary guide base and provides a landing base for additional strings of casing. A permanent guide base is secured around the conductor housing assembly and is lowered simultaneously with the foundation pile.
The permanent guide base comprises a foundation structure of approximately the same dimensions as the temporary guide base and includes a central opening for receiving the conductor housing assembly. The permanent guide base further includes four guideposts extending upwardly from the structure for receiving the four guidelines extending from the temporary guide base. Once the guidelines have been inserted into slots in the guideposts at the platform, the permanent guide base with attached foundation pile is lowered on a string of pipe into position atop the temporary guide base on the ocean floor. The two guide bases include cooperative structure, sometimes referred to as a gimbal structure, for maintaining the permanent guide base level in a horizontal plane despite as much as a twelve degree deviation from the horizontal in the position of the temporary guide base due to a sloping ocean floor.
Once the permanent guide base and foundation pile are set in place and cemented, additional drilling and casing occurs, a wellhead and a subsea blowout preventer stack are installed, and deeper drilling begins in earnest. The permanent and temporary guide bases provide primary guidance and support for these activities.
When all drilling activity has been completed and the usefulness of the drilled borehole is expended, clean-up of the drilling site begins. In many parts of the world, for example, in the North Sea off the coast of Norway, the ocean floor in the area of the drilling site must be cleared of all drilling apparatus. Thus, casing is cut from within the borehole and tripped back to the platform at the ocean surface. The foundation pile is severed with explosives and tripped to the platform along with the permanent guide base. Finally, the temporary guide base is retrieved using a running tool on a string of drill pipe.
Often, however, there is some difficulty in retrieving the temporary guide base. Depending on soil conditions at the ocean floor, the temporary guide base may have become substantially buried. Cuttings dumped onto the guide base during initial drilling operations may contribute to the problem. The guide base is normally retrieved with the aforementioned running tool by engaging J-shaped slots within an inner sleeve in the central opening of the guide base. When the temporary guide base is even partially buried, however, the process of engaging the J-slots with the running tool can prove to be quite difficult and time-consuming. The problem is compounded if the temporary guide base rests on an incline.
The difficulties associated with the conventional method for retrieval of the temporary guide base gave rise to an outcry among companies engaged in offshore drilling for a more efficient method of retrieval. Initial attempts at solving the problem have focused on a hook apparatus for securing the temporary guide base to the permanent guide base when the latter is lowered onto the former. Thus, when the permanent guide base and foundation pile are tripped to the surface, the temporary guide base is simultaneously retrieved.
Generally, such hook apparatus have been successful in achieving simultaneous retrieval of the temporary guide base. However, such hook apparatus tend to limit movement of the permanent guide base relative to the temporary guide base, thus restricting the operation of the gimbal structure and, in some cases, preventing the permanent guide base from assuming a horizontal position atop an inclined temporary guide base.
Hence, it appears that a need exists for a simple and efficient means for retrieving the temporary guide base. It further appears that the known prior art does not provide suitable means for addressing this need.